Geodynamics and Ore Content of Proterozoic Maphites in the Central Part of the Aldan-Stanovoy Shield (Southern North Asian Craton)

1. INTRODUCTION

The Paleoproterozoic accretionary and collisional structures (2.2–1.6 Ga) are characterized by the combination of pyrite-polymetallic, gold and copper-nickel-platinum-metallic ores [Turchenko, 2021]. The exposed Nimnyr terrane within the Aldan-Stanovoy Shield (Fig. 1) is no exception. The Precambrian geological complex terranes interpreted as the root zones of the Paleoproterozoic orogenic belt [Smelov, Timofeev, 2003] are known for the Precambrian metamorphogenic gold [Syasko et al., 2006] and skarn deposit with high Fe, Mn, Cu, and Co concentrations [Parfenov, Kuzmin, 2001]. The placer gold deposits often reveal a platinum-sperrylite association. Thus, the Unga-Nimgerkan placer deposits are characterized by the presence of rounded sperrylite grains, often with a thin surface coating of native platinum, which contain small kaolinite, pyrophyllite, endellite, phlogopite, ore mineral and quartz inclusions. The studies of structural features and mineral inclusions showed that such intergrowths were formed during thermal metamorphic decomposition of sperrylite at temperatures between 400 and 600 °C with participation of hydroxyl-containing minerals [Okrugin, 2000]. The ¹⁹⁰Pt-⁴He age of three sperrylite grains varies from 1.70 to 1.95 Ga [Okrugin et al., 2018, 2020].

The study was aimed at forecasting potential indigenous sources of gold and platinum which form placer and primary deposits of the area. This requires the development of objective mineralogical-geochemical criteria for certain parameters of formation conditions for deep-seated sources of parent ore. The tasks included: 1) determination of chemical composition and content of admixture elements in metabasites and dolerites; 2) identification and characterization of rocks based on the petrographic and geochemical data; 3) analysis of distribution patterns for chemical elements and geochemical indicator ratios of geodynamical settings and sources of the substance; 4) ore mineralization formation model refinement.

2. GEOLOGICAL STRUCTURE OF THE STUDY AREA

The outcrops studied are located within the Nimnyr granulite-orthogneissic terrane (Fig. 1). The structural plan of the terrane is determined by the widespread occurrence of granite-gneiss domes. The largest of them – the Timpton dome, – is located in the northern part of the terrane [Duk et al., 1986]. The domal cores are composed of 3.57–1.93 Ga orthogneisses represented by granite-, charnokite- and enderbite-gneisses with amphibolite bodies [Parfenov, Kuzmin, 2001]. The shoulders of the domes are composed of the paragneissic complex represented by two associations of rocks. The first association (Kurumkan sequence) includes quartzites and high-alumina gneisses, protolithically resulted from decomposition of the rocks with Nd-model age of 3.06–2.85 Ga [Parfenov, Kuzmin, 2001], as well as calcareous and ferruginous quartzite lenses [Rundqvist, Mitrofanov, 1988; Duk et al., 1986]. The second association (Fedorovka sequence) is represented by 2006 Ma amphibole, biotite-amphibole, diopside-amphibole and bipyroxene-amphibole plagiogneisses [Velikoslavinsky et al., 2006], less often by schists with interlayers and lenses of diopside and phlogopite-diopside rocks and calciphyres [Parfenov, Kuzmin, 2001] (Fig. 2).

Gold mineralization in the terrane is localized in extended metabasite bodies of the Medvedev complex, intruding sub-conformably the orthogneissic complex and metamorphic rocks of the Kurumkan and Fedorovka sequences and constituting combined dikes with 1920 Ma alaskite granites [Shcherbak, Bibikova, 1984; Kravchenko et al., 2009]. The bodies are located in the interdomal synforms (Fig. 3, a). The degree of metamorphism experienced by metabasites and host rocks corresponds to the granulite facies. Among the rocks of the Medvedev complex, the most common are bipyroxene-amphibole and pyroxene-amphibole crystalline schists; there occur olivine- and pyroxene-containing amphibolites. At some outcrops, the bodies are deformed by asymmetric folds with steep hinges which occur during the shear motions. Where the metamorphic rocks show signs of bending, folding and secondary schistosity in association with sub-conformable quartz veins and pyroxene and amphibole bands and lenses, the crystalline schists contain vein-disseminated metamorphogenic-hydrothermal sulfide-arsenide ores presenting in the Pinigin deposit and some ore fields (Fig. 3, a, b). It suggests the formation of the complex and its related mineralization at the final collisional stage [Smelov et al., 2006; Kravchenko et al., 2010]. The ⁴⁰Ar/³⁹Ar age of metamorphism of the rocks from the Medvedev complex, determined based on the amphiboles from the bipyroxene-amphibole crystalline schist and pyroxene-plagioclase-quartz rock in the ore interval, was 1903–1908 Ma [Smelov et al., 2006].

The formation temperature estimation values for bipyroxene and amphibole-plagioclase associations of the crystalline schists of the deposits using different geothermometers were 750–850 °C; pressure estimation value based on the amphibole composition was 5–7 kbar [Kravchenko et al., 2010]. Based on the fluid inclusions in plagioclases and pyroxenes, there were obtained the effects of gas emission corresponding to high-temperature (780–480 °C) and moderate-temperature (450–180 °C) ore-forming fluids of the metamorphic and later stages [Sharova, 2006].

The area of occurrence of metabasites of the Medvedev complex includes a zone of intersecting doleritic dikes (Fig. 3), widespread within the Aldan Shield and constituting the E-NE- and NW-extended dike belts [Okrugin et al., 2000, 2019] – Timpton-Gynym (TG) and Timpton-Algamay (TA), with precise U-Pb ages of 1869±2 and 1754±5 – 1759±4 Ma [Ernst et al., 2016]. The metabasites of the Medvedev complex and ore bodies are intersected by dolerites (Fig. 3, b). The doleritic dikes are steeply dipping (70–90°), brought into direct, sharply defined transcurrent contacts, extend a few hundred meters to 12–15 km, and vary in thickness from a few tens of meters to 200–300 m. They are unsusceptible to metamorphic transformations, fresh-appearing, and consist of dark-gray and black massive fine- and medium-grained rocks with fine-grained chilled endocontact zones. The rocks have ophitic and gabbro-ophitic texture; at the endocontacts, they are characterized by microlithic texture. The primary minerals are plagioclase and clinopyroxene, the secondary minerals are K-feldspar, hypersthene, quartz, titanomagnetite, and apatite. There are dioritic dikes characterized by a high content of interstitial quartz-feldspathic micropegmatite and the occurrence of hypersthene. The 1869 Ma dikes were intruded at the post-collisional extension stage, and the 1750 Ma dikes reflect the intraconinental extension stage [Donskaya, Gladkochub, 2021; Gladkochub et al., 2022].

The belonging of metabasites and dolerites to the pre-ore and post-ore magmatic rocks of similar material composition makes them actual for investigation of mineralogical-geochemical features indicating ore genesis processes.

3. METHODS AND MATERIALS

On the Pinigin deposit area, eastward to the Sivaki and Korot rivers, westward to the Khair and Khatymi rivers and northward near the Orto-Sala and Seligdar rivers, there were collected 55 rock samples from the Medvedev complex and more than 40 dolerite samples from 8 dikes of the Timpton-Gynym and Timpton-Algamay belts.

Optical properties of minerals and rocks were studied using Meiji Techno 9430L, Olympus Bx50 and Polam-Р-211 microscopes. 75 samples were used to determine chemical composition and microelements including REE. The contents of rock-forming oxides were determined by silicate analysis using wet chemical methods at the department of physical and chemical methods of analysis of DPMGI SB RAS (Yakutsk). The rare-earth and rare element contents were analyzed by LA-ICP-MS at the Shared Research Facilities of Multi-Element and Isotope Researches of SB RAS (IGM SB RAS, Novosibirsk). The analytical data were processed using comparative characteristics and discriminant and petrogenetic diagrams drawn by different authors.

4. RESULTS

4.1. Rock composition in the Medvedev complex

Five groups of rocks within the Medvedev complex can be distinguished according to the petrographic data. The first and the second groups are represented by hornblende-rich amphibolites which also contain some amounts of olivine and pyroxene. Individual samples include spinel and magnetite. A characteristic distinction of the second group of amphibolites is the presence of a small amount of plagioclase. The third-fifth groups of rocks are dominated by pyroxene-amphibole and bipyroxene-amphibole crystalline schists. Some of the schists contain rare ore minerals, titanite, apatite, biotite, and quartz. The fourth and the fifth groups are characterized by the presence of ore minerals and pyroxene. All samples of the fifths group contain clinopyroxene. Gold mineralization is confined to the last two groups. The rocks of the groups in their chemical composition (Table 1; Suppl. 1 on the article page online) are similar to: 1 – peridotites, pyroxenites and hornblendites; 2 – pycrodolerites; 3 – high- and moderate-Mg dolerites; 4 – low-Mg dolerites; 5 – (high-ferriferous) dolerites.

The non-metamorphosed post-ore doleritic dikes possess homogeneous structures with no visible signs of differentiation. However, they differ from each other in chemical composition (Table 1; Suppl. 1 on the article page online). An important discriminant feature is Ti- and P-contents relative to alkalinity and SiO₂-content [Okrugin et al., 2000]. According to this, the dikes are divided into two groups – Ti- (TiO₂ more than 1.5 wt. %) and low-Ti (TiO₂ less 1.5 wt. %) [Okrugin et al., 2000, 2019].

A feature common to the rocks the Medvedev complex or dolerites is that they belong to the normal range of alkalinity and tholeiitic series (Fig. 4, a–c). In terms of rock-forming oxide content, there is no overlap in composition between the metabasites of groups 1–3 and dolerites; the metabasites of groups 4-5 are similar to Ti-dolerites (Fig. 4, a–d). The points of low-Ti- and Ti-dolerites and NW- and NE-trending belts in Fig. 4 coincide with each other, i.e., there is no difference between these belts. Ti-dolerites are characterized by a high content of P₂O₅ (0.2–0.5 wt. %); a similar trend is also seen for the change in TiO₂ and P₂O₅ contents depending on silica content in the rocks (Fig. 4, d). Low-Ti dolerites with a high SiO₂ content show a gradual increase in Ti- and P-contents while Ti-dolerites exhibit abrupt rather than gradual decrease in the content of these elements. This duality of titanium and phosphorus behavior may be attributed to fractional differentiation of basite melts in deep-seated chambers. Thus, the dolerites exhibit only geochemical (low-Ti and Ti) characteristics which probably resulted from differentiation of basites in the intermediate sources with no dependency on the age of different-trending dike belts.

Spectral distribution of elements on the spider diagram shows that the metabasites of groups 1–3 are similar to enriched mid-ocean ridge basalts and those of groups 4–5– to intraplate ocean island basalts. Unlike the latter, the dolerites are characterized by the presence of well-defined negative Th-U, Nb-Ta, Zr-Hf, P and Ti anomalies (Fig. 4, e). In the discriminant diagram, reflecting compositional features of rocks in different geodynamic settings, most of the metabasite and dolerite composition points fall into the fields corresponding to intraplate tholeiites, combined with intraplate alkaline and arc volcano basalts (Fig. 4, f).

Spectral distributions of heavy rare-earth elements are different. It was found that REEs present relatively low contents in metabasites of groups 1–3. With an increase in Al₂O₃ content, there is an observable increase in the REE spectral slope (Gd/Yb, Dy/Yb ratios in Table 1) and a change of negative Eu anomalies to positive (Fig. 5, a). These features can be indicative of the process of magmatic differentiation with possible participation of plagioclase. The calculation of rare-earth element contents during the melting of clinopyroxene-rich lherzolites [Lesnov, 2010] using batch melting equation C_L=C₀/(D₀+F(1-P)) [Zou, 2007] shows that the REE contents similar to those for groups 1–3 can be obtained on 30, 10 and 1 % melting in these rocks (Fig. 5, d).

More gentle spectral slopes and high rare-earth element contents were found in metabasites of groups 4–5 (Fig. 5, b). The calculation of rare-earth element contents on melting of harzburgites [Lesnov, 2010] shows that the REE distribution similar to that in groups 4 and 5 will most likely be obtained by 1 and 10 % melting in these rocks (Fig. 5, e). The calculations of the REE contents on melting of garnet lherzolites show other spectra, though high heavy REE contents could be attributed to the presence of garnet [Lesnov, 2012]. The negative and positive Eu anomalies are probably caused by the process of magmatic fractionation.

Dolerites show HREE concentrations comparable to those in metabasites but higher LREE concentrations (Fig. 5, c). This can be related to the participation of an enriched mantle source in the initial melt formation. There were detected weak negative Eu anomalies. The low-Ti dolerites exhibit a more gently sloping HREE distribution pattern (Gd/Yb, Dy/Yb ratios in Table 1).

The influence of different degrees of melting of the mantle sources on the rock compositions can be seen in diagrams (Fig. 5, f, g). Flat HREE distribution patterns of rocks in these diagrams are confined to the fields reflecting a higher melting degree.

The analysis of the diagrams reflecting the source types shows that the enrichment of metabasites and dolerites in rare and rare-earth elements is of a different nature. The metabasites are characterized by geochemical markers related to non-subduction, probably plume component, and the dolerites – by the markers related to subduction-enriched lithospheric mantle (Fig. 6, a–e). During the initial melt formation the protoliths of the rocks from the Medvedev complex were presumably derived from different-degree mantle melting and plume-lithosphere interaction which is evidenced by the point distributions along melting and assimilation trends in Fig. 6, d, e.

4.2. Material composition of ores

The associations of ore minerals in metabasites are related to magmatic, metamorphic and hydrothermal processes. There were found the relics of magmatic pentlandite-chalcopyrite-pyrrhotite ores having an association of cubic chalcopyrite – hexagonal nickel-bearing pyrrhotite with decay structures (Fig. 7, a). Metamorphogenic pyrrhotite (without decay structures), chalcopyrite, arsenopyrite and loellengite were found with pyroxenes, hornblende, and plagioclase (Fig. 7, b–d). The shape of the above-mentioned mineral grains reflects the structural-balance (Fig. 7, b) and induction (Fig. 7, d) joint growth boundaries typical of metamorphogenic structures. The content of gold (admixture form) in loellingite is up to 200 ppm. The replacement of rock-forming minerals by biotite, actinolite, chlorite, quartz veinlets and rare K-feldspar grains is associated with hydrothermal generation of pyrrhotite and arsenopyriye (Fig. 7, e–i). The latter are represented by homogenous small grains occurring in bunches and veinlets. Arsenopyrite occurs as idiomorphic crystals in hydrothermal association of ore minerals together with pyrrhotite and chalcopyrite, forming either single grains or small aggregates. Cobaltite and isolated occurrences of native gold precipitation form rims around loellingite (Fig. 7, f). A microprobe analysis showed the presence of bismuth and tellurium minerals, as well as of stibnite and sсheelite (Fig. 7, g–i).

5. DISCUSSION

In terms of formation conditions and set of wallrocks and ores, ore mineralization in metabasites of the Medvedev complex can be compared with hypozonal orogenic gold deposits (Fig. 8, a, b). Such deposits are characterized by the early ore stage dominated by pyrrhotite, arsenopyrite, and loellingite. It is followed by the late ore stage which comprises gold, non-precious metals, and Bi-containing minerals. Mineral ore deposits form at ~4.0–5.5 kbar and 450–550 °С [Groves et al., 2020a; Zhao et al., 2022; Li et al., 2022]. This type of deposits is usually characterized by the ore-forming fluid source problem [Fu, Touret, 2014; Phillips, Powell, 2009] (Fig. 8, b). The approach to solving this problem traditionally emphasizes subduction processes (Fig. 8, c); an alternative model is related to the asthenospheric upwelling and reactivation of the ancient enriched lithospheric mantle [Groves et al., 2020a] (Fig. 8, d).

The culmination accretionary-collisional, metamorphic and magmatic events in the central Aldan Stanovoy Shield are confined to the Paleoproterozoic (2.01–1.87 Ga) [Kotov, 2003; Donskaya, 2020; Smelov, Timofeev, 2007]. From 2011±2 to 2006±3 Ma, there was an initiation of the Fedorov island arc and formation of volcanic rocks of the Fedorovka sequence. 1993±1 Ma is associated with the formation of the Chuga and Fedorovka thrust faults as a result of the collision between the Fedorovka volcanic arc and the Olekma-Aldan continental microplate [Velikoslavinsky et al., 2006; Kotov, 2003; Anisimova, 2007]; 1.97–1.95 Ga – with the formation of granite intrusions and leucogranites of the Dzhaltundin complex which are post-collisional relative to the collision of the Fedorovka island arc [Kotov, 2003].

It is probable that the formation of the subduction-enriched lithospheric mantle is related to the Fedorovka rock sequence development and occurred prior to the formation of rocks of the Medvedev complex.

The time interval of 1.95–1.92 Ga for the Nimnyr terrane is characterized by the eastward-observed formation of an active margin of the Olekma-Aldan continental microplate and the accumulation of volcano-sedimentary sequences east of the terrane [Kotov, 2003]. Based on the study of xenoliths from the Precambrian metamorphic complexes in the Mesozoic intrusives, 1.96–1.90 Ga terrane is presumably associated with magmatic underplating [Kravchenko et al., 2012]. This age overlaps with the intrusion [Kravchenko et al., 2009] of 1.92 Ga dikes of the Medvedev complex and 1.91–1.90 Ga metamorphism. Metamorphism is characterized by the isothermal decompression trend [Smelov, 1996]. 1.87 Ga ago there was the formation of granite intrusions of the Kodar complex, most likely corresponding to the final stages of the processes during collision of the Aldan and Dzhugdzhur-Stanovoi continental plates [Kotov, 2003], and post-collisional dolerite intrusion.

Ore formation in the rocks of the Medvedev complex lasted from 1.92 to 1.87 Ga. There is no evidence for subduction during this interval; it is characterized by intrusion and metamorphism of pre-ore basites of the Medvedev complex and formation of post-ore non-metamorphosed doleritic dikes. During the same interval, the metamorphic complexes were probably brought to higher levels. The above-described geochemical features of plume activity and enriched lithospheric mantle melting, as well as the features of melt assimilation, mean there is a probability of the plume-lithosphere interaction and ore material activation. The features revealed for the compositions of the mantle magmatic complexes are more suitable for the model shown in Fig. 8, d. Geophysical field imaging of domal structures of the Nimnyr terrane (Fig. 9 a, b) and melt flow channels in the mantle (Fig. 9, c) confirm the adequacy of this model.

Attention is also drawn to a widespread occurrence of placer sperrylite and platinum whose mineralogical-geochemical features are indicative of potential platinum-metal mineralization in this area, which does not exclude the possibility of the Paleoproterozoic complex precious metal deposition.

Multiple mantle magmatic complexes and complex geological structure of the study area necessitate further investigation of mantle events to specify mechanisms and models of ore formation. A more accurate characteristic of sources and geodynamic processes requires the representative isotope-geochemical and isotope-geochronological studies.

6. CONCLUSION

The compositional features of pre-ore and post-ore mantle magmatic complexes imply that the most probable source of the Paleoproterozoic ore-forming fluids in the Nimnyr terrane was the ancient subduction-enriched lithospheric mantle interacting with the asthenospheric material. The metamorphic complex formation and its related mineralization occurred at the final collisional stage which favored concentration of ore and its deposition on fold and secondary schistosity sections in metabasites. The period from 1.92 to 1.87 Ga, between formation times of granulite-facies pre-ore basites and post-ore non-metamorphosed dolerites, is likely to be characterized by a significant decrease in the depth of occurrence of metamorphic complexes and by ore formation.

7. ACKNOWLEDGEMENTS

The authors are grateful to their colleagues who took part in field and laboratory research, O.M. Turkina and T.V. Donskaya for their comments.

8. CONTRIBUTION OF THE AUTHORS

All authors made an equivalent contribution to this article, read and approved the final manuscript.

9. DISCLOSURE

The authors declare that they have no conflicts of interest relevant to this manuscript.